In battlefield conditions it is often desirable to remotely identify an object as an item of interest. This is typically done using a hand-held or otherwise portable Laser Designator or Laser Rangefinder (LRF) to illuminate the object with laser energy such that an appropriate sensor system can detect the illumination and identify the object.
Operators of Designator systems have an operational need to determine the specific location being illuminated by the laser radiation (the laser spot). In the current state of the art, this task is complicated by several factors, not the least of which is that laser illumination sources typically utilize wavelengths invisible to the human eye in order to function covertly. Thus, it is imperative that laser designator systems provide a means for the operator to confirm that the correct object is being illuminated.
Optical reticles incorporated into a camera system are often used to provide an indirect visual indication of the direction of laser illumination. The propagation direction of the laser source is aligned (boresighted) to the line-of-sight of the reticle, such that the point observed at the center of the reticle is also the point illuminated by the laser. Since the laser illumination is not visible, the operator relies on boresight alignment being maintained to ensure that the laser energy is deposited at the location observed at the center of the reticle. Any angular boresight error results in the laser output being directed in a direction different than where the reticle is looking. This results the laser illuminating the incorrect location, a condition which is highly undesirable. Further, since the laser energy is not visible, this condition is not readily detectable by the operator.
Earlier laser trackers employed quadrant detectors to directly detect the reflected energy from the laser spot as a means to determine the angular direction of a laser-illuminated object relative to the line-of-sight of the Designator. This approach provided some assurance that the correct object was being illuminated.
A quadrant detector is a device containing four individual sensing elements arranged around a common center point. Determination of angular direction using a quadrant detector is accomplished by first measuring the intensity of the reflected laser light from the object of interest with incident on each of the four sensing elements within the detector. The relative amplitudes of the individual signals from the sensing elements are then used to determine the angular direction of the object. As an example, if the quadrant detector line-of-sight is pointing directly at the object then each of the four sensing elements will receive equal energy reflected from the object, and consequently will have equal output signals. Any deviation of the object off of the detector line-of-sight will result in an imbalance in the outputs from the sensing elements, which provides an indication of the direction to the object.
While trackers utilizing quadrant detectors are accurate under ideal conditions, they present a number of challenges in an operational environment; in particular a very high Signal to Noise Ratio (SNR) is required for accurate determination of the angular direction of the object relative to the tracker optical axis. In order to determine the angular location of an object at typical distances of three to four kilometers from the tracker with sufficient accuracy, a laser with high power output is required. Use of such a laser can be problematic due to size and weight of the laser and associated power supply, and because a bright, high power laser source is readily detectable, an undesirable condition in many applications.
Current systems require illumination by a laser source emitting either a continuous series of pulses or a constant output. Both approaches degrade the ability to operate covertly by providing an adversary more opportunity to detect the source of the laser transmission than that minimally provided by single-pulse operation.
A need exists to covertly illuminate an object using a single laser pulse. A single, short duration laser pulse would be directed at the object of interest, and the reflected signal detected in such a way as to provide an indication of the direction to the object. This allows the operator to determine the exact location of the laser spot while minimizing the ability of an adversary to detect the location of the laser source.
The See-Spot invention described herein avoids all of the issues inherent in current state-of-the-art Designators by providing direct visual confirmation of the location being illuminated, while using low power laser sources in single-pulse operation.